1. Field of the Invention
This invention relates generally to composite materials.
This invention relates particularly to the preparation of composite materials wherein the matrix materials are tough aromatic thermoplastic polyimides that exhibit unusually low viscosities during the application of heat.
2. Prior Art
Polyimides are attractive to the aerospace industry because of their toughness, thermal and thermooxidative stability, solvent resistance and excellent mechanical and electrical properties over a wide temperature range. However, processing of these materials has been difficult due to limited melt flow during the application of heat and pressure. It has been shown that the substitution of aliphatic segments into aromatic polyimides results in the reduction of the softening temperature of modified polyimide polymers; however, the thermooxidative stability of the system was comprised. V. L. Bell et al utilized meta-substituted aromatic diamines in the preparation of homopolyimides, which rendered the polymer thermoplastic at low temperatures and moldable at moderate to high pressures. Adhesive and film forms of these polymers were also developed, as well as resins suitable for impregnation of carbon fibers. However, the production of structurally sound, void-free laminates from prepregs was difficult due to the poor melt flow characteristics exhibited during processing.